1. Field of the Invention
The invention relates to an optical system, and more particularly, to an optical touch system.
2. Description of Related Art
In the conventional touch rear projection system, an infrared beam irradiates on a transparent screen by an infrared light emitting diode (IR-LED). When a finger or a tip of a touch pen touches the screen, the infrared beam is reflected to an image detector. Through the infrared image sensed by the image detector, the control unit electrically connected to the image detector may determine the position of the finger or the tip of the touch pen relative to the screen.
Because the IR-LED and the image detector are disposed at the same side of the screen, generally, at least one ray of the infrared beam emitted by the IR-LED may not pass through the screen, and the ray is reflected to the image detector by a surface of the screen. Because the ray reflected by the screen follows law of reflection, i.e. the incident angle of the ray striking the screen is equal to the emergence angle of the ray leaving the screen, the ray has a certain intensity, and forms an over exposed hot spot on the image detector.
In this manner, whether the position on the screen irradiated by the light is touched by the finger or the tip of the touch pen or not, the hot spot exists in the corresponding position on the image detector. So that the control unit may be unable to determine whether the position is touched by the finger or the tip of the touch pen.
In order to solve this issue, in the conventional touch rear projection system, the infrared beam emitted by the IR-LED is emitted to the screen by a larger angle, so that the light of the infrared beam reflected by the screen and following law of reflection may not enter the image detector. However, the volume of the touch rear projection system may become larger. Furthermore, because the over large incident angle of the infrared beam may cause the insufficient illumination, a plurality of IR-LEDs or an IR-LED capable of outputting higher energy may be adopted, and waste energy. Moreover, adopting too more IR-LEDs also correspondingly increases the number of the image detector, thereby increasing the number of the image capture lens of the image detector, and thus, enhancing the cost.
Besides, Taiwan patent no. M355612 discloses an infrared light source, an infrared image capture lens, a screen, a reflection unit, and a reflecting mirror.